Bounder deposing the king of the castle (3)
I believe the answer is:
roo
'bounder' is the definition.
(short for kangaroo, something which bounds)
'deposing the king of the castle' is the wordplay.
'deposing' indicates named letters should be taken away.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'of' becomes 'o''.
'castle' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'r'+'o'+'the'='rothe'
'rothe' with 'the' taken away is 'ro'.
'ro'+'o'='ROO'
Can you help me to learn more?
(Other definitions for roo that I've seen before include "Kanga's child (Milne)" , "Young marsupial" , "Kangaroo (Austr. colloq.)" , "Wild animal" , "companion of Pooh" .)